A typical hydraulic power steering system for a vehicle includes an engine-driven hydraulic fluid pump having an inlet port connected in fluid communication with a hydraulic fluid reservoir. The pump has an outlet port connected in fluid communication through a main conduit with a power steering valve. The typical vehicle hydraulic power steering system also includes a power steering motor operatively connected with steerable vehicle wheels. When the steering valve is actuated, it directs fluid flow from the pump to the power steering motor. The power steering motor then operates to steer the vehicle wheels. The power steering valve may be an open-center valve having a neutral position in which fluid flow from the pump flows through the steering valve and back to the fluid reservoir.
The power steering pump is driven by the vehicle engine. Horsepower is unnecessarily consumed when excess fluid from the pump flows through an open-center steering valve, for example in a nonsteering condition of the vehicle. A known hydraulic power steering system also includes a bypass valve which bypasses fluid from the main conduit to the fluid reservoir. An orifice is usually located in the main conduit to provide a hydraulic fluid pressure signal which varies as the amount of fluid flow through the orifice varies. The hydraulic fluid pressure signal from the orifice communicates with the bypass valve to control operation of the bypass valve. The bypass valve is controlled to direct excess fluid flow from the pump to the reservoir. When the bypass valve directs excess fluid flow from the pump to the reservoir, the excess fluid does not flow through the steering valve. This results in minimizing horsepower consumption. U.S. Pat. No. 4,691,797 discloses a typical hydraulic power steering system having a bypass valve.